The invention relates to a projection exposure system, in particular for microlithography, comprising a catadioptric projection objective and a light source.
In such projection exposure systems, illumination-induced imaging errors occur that are due to the thermal deformation of the optical components present-in the projection objective. The thermal deformation results in this case from the heating of the optical components occurring as a result of the residual absorption of the projection light.
A further illumination-induced effect that results in the imaging errors is the refractive index change due to the illumination light in the transilluminated material of the optical components. Such refractive index change effects may occur reversibly, with the result that the refractive index of the optical components is the same after the irradiation as before, or they may also be irreversible.
Said illumination-induced imaging errors impair the imaging quality of the projection exposure system and cannot be accepted, in particular, at those points where the resolution of very fine structures is desired.
The object of the present invention is therefore to develop a projection exposure system of the type mentioned at the outset in such a way that illumination-induced imaging errors are reduced.
According to the invention this object is achieved in that
a) at least one mirror and at least one lens assigned to it of the projection objective are composed of materials chosen in such a way as a function of a given light intensity distribution in the projection objective and
b) the position of the mirror and the position of the lens are similar inside the projection objective in such a way
c) that the imaging changes in the projection objective that are due to an illumination-induced imaging change in the reflection surface of the mirror counteract illumination-induced imaging changes in the lenses.
The invention is based on the observation that changes in the radius of curvature of optical components, that is to say, for example, a reduction in the radius of curvature of a concave optical surface, have a different effect in the case of a reflecting surface on the optical imaging properties of said surface than in the case of a refracting surface.
In particular, a lens (or a lens group) and a mirror that are disposed in a similar position inside a projection objective and either both have a collecting or both have a divergent effect counteract one another because of the imaging changes with regard to their imaging properties that occur in them as a result of the illumination-induced heating. This results in the possibility of bringing about compensation for the illumination-induced imaging changes. xe2x80x9cSimilar positionxe2x80x9d is understood as meaning such a positional assignment of the reflecting surface with respect to the refracting surface, that is to say of the mirror with respect to the at least one lens that the subaperture ratios in the reflecting and in the refracting surface do not differ considerably. In this connection, the subaperture ratio is the ratio between the distance between diametrically opposite impingement points of peripheral rays that proceed from a field point, that is to say from a point in the object to be projected, on the optical surface and the unobstructed aperture of said optical surface inside the projection objective.
For a given lens design of a projection objective, a lens or a lens group whose subaperture ratio does not differ considerably from that of the mirror can as a rule be specified for a mirror inside the projection objective. A selection of the materials that can be supported by a calculation and from which the mirror and the lens or the lens group are to be made results in total in imaging properties of the projection objective that depend only to a small extent or not at all on illumination-induced changes in the imaging properties of the individual optical components.
In this connection, the material is selected so as to take account, for example, of the thermal conductivity, of the coefficient of thermal expansion and of the refractive index behaviour during a temperature change in the respective material. In addition, the refractive index behaviour of the respective material can be taken into account as a function of the illumination intensity.
Such compensating projection objectives are to a large extent independent of illumination-induced drift of the imaging properties of the individual components. This increases the achievable throughput of the projection exposure system.
Preferably, subaperture ratios of the mirror and of the lens differ at their respective positions inside the projection objective by less than 25%. This ensures that the imaging beams that are assigned to the individual object points are influenced in the same way by the assigned optical surfaces, with the result that as great a compensation as possible can take place of imaging errors that are illumination-induced in the individual components.
More than one lens may be assigned to the mirror. Owing to the optical conditions during reflection, dimensional changes in a reflecting surface result in greater imaging changes than identical dimensional changes in a refracting surface. It is therefore advantageous to use a plurality of lenses to compensate for the illumination-induced imaging changes in a mirror. Since, as a rule, the number of lenses in known catadioptric projection objectives is substantially greater than the number of reflecting surfaces, such an assignment can be carried out, as a rule, without fairly large changes to the optical design.
Preferably, precisely two or precisely three lenses are assigned to the mirror. In known designs of a projection objective, a lens group that comprises two individual lenses in one design and three individual lenses in another design is disposed inside a projection objective in such a way that it differs in its subaperture ratio only very slightly from that of the mirror.
Said lens group is therefore particularly well suited for compensating for illumination-induced imaging changes that are due to the heating of the mirror surface. In addition, the lens group in these known projection objectives is disposed directly adjacently to the mirror so that they can form a unified assembly with the latter. This simplifies the retrofitting of the known projection objectives with an optical assembly that compensates in the above sense.
The lens may have a lens body composed of CaF2. Calculations have shown that the use of lenses composed of said lens material together with conventional materials that are used for mirrors in catadioptric projection objectives result in a good compensation for imaging errors.
The mirror may have a mirror base composed of one of the glass materials BK3, BK6 or BK7. These materials are robust, can readily be machined and have thermal properties that make them appear well suited for use as a compensating element.
Alternatively, the mirror base may be constructed of the glass material SK1. The coefficient of thermal expansion of SK1 is between BK3 and BK7, with the result that this material recommends itself for certain applications.
In a further alternative embodiment, the mirror may have a mirror base composed of one of the glass materials FK51 or FK54. These materials have relatively high coefficients of thermal expansion compared with the abovementioned glasses and therefore offer the potential of a large optical compensation effect for illumination-induced imaging errors at least of a mirror.
Alternatively, the mirror may have a glass support composed of the transparent glass ceramic material marketed under the trademark name Zerodur(copyright). Zerodur(copyright) is a glass that has an extremely low coefficient of thermal expansion since it is composed of crystalline and amorphous constituents. Illumination-induced effects that result from the heating of Zerodur(copyright) are therefore very small. In the case of such a Zerodur(copyright) mirror, the illumination-induced imaging changes are therefore small and provide, for example, the possibility of undertaking a compensation with only one individual lens.
Depending on the production process, a low coefficient of thermal expansion can be achieved in the case of Zerodur(copyright) with positive or negative sign. In the case of negative coefficient of thermal expansion, the imaging change base on an illumination-induced dimensional change in a mirror composed of this material operates additively with respect to the illumination-induced change in the imaging properties based on a dimensional change in an assigned lens. This material is therefore suitable for use in cases in which changes in the imaging properties result in an overcompensation of the illumination-induced dimensional change in the lens owing to an illumination-induced refractive index change in the lens material. In the case of the lens, this results in a net effect of the illumination-induced imaging change that counteracts the illumination induced imaging change in the mirror.
Finally, the mirror may alternatively have a mirror base composed of monocrystalline silicon. This material also has a low coefficient of thermal expansion compared with the glasses. This again results in only very small illumination-induced imaging changes in the mirror.
The projection exposure system can be operated with a conventional illumination setting. Such an illumination setting, that is to say a xe2x80x9chomogeneous fillingxe2x80x9d with projection light in a pupillary plane of the projection objective, results, as a rule, in a heating of the optical components of the projection objective, which is greater in the center of the illuminated surface than at the periphery. This reduces the radius of curvature of convex optical surfaces involved and increases the radius of curvature of concave optical surfaces involved. Calculations have shown that such changes in the radius of curvature can readily be compensated for by a combination of a mirror and a lens or lens group assigned to it within the projection objective.
Alternatively, a coherent illumination setting may be used. Such an illumination setting corresponds to a xe2x80x9chomogeneous fillingxe2x80x9d with illumination light only in a central region of a pupillary plane of the projection objective. The change in the radii of curvature of the optical surfaces involved is therefore restricted to said central region. In other respects, however, similar conditions arise as in the case of the conventional illumination setting, with the result that a good compensation is also possible.
A third illumination setting that can be used is an annular illumination setting. This is characterized by an xe2x80x9cannular fillingxe2x80x9d in a pupillary plane of the projection objective, in which a central region is not illuminated. In an annular region around the optical axis of the optical-surfaces, this results in a reduction in the radius of curvature in the case of convex surfaces and an increase in the radius of curvature in the case of concave surfaces. Here, again, calculations show that illumination-induced imaging changes that are due to a mirror can readily be compensated for by an assigned lens or lense group.
A heating device for the mirror and/or the lens can be provided to additionally compensate for the illumination-induced imaging changes. Such a heating device can generate a compensating shaping of an optical surface within the projection objective that further improves the compensation for the illumination-induced imaging changes. All in all, such a projection objective can be manufactured in such a way that it is substantially neutral in its imaging properties, that is to say independent of the projection illumination.
The heating device may comprise a light-absorbing coating of the mirror and/or of the lens. In this case, the illumination light itself provides for the additional compensating action described in that a specified heating and, consequently, a specified deformation of the coated optical components takes place as a result of an absorption extending beyond the unavoidable residual absorption. The deformation can be influenced by the thickness, the specific absorption coefficient and the position of the coating on the respective optical surface.
Alternatively or additionally, the heating device may comprise at least one active heating element that is disposed at the circumference of the lens and/or of the mirror. In this way, an additional, controlled heating input can take place into the lens or mirror body that results in a defined dimensional change. This can be used for additionally compensating for illumination-induced imaging changes.
The heating device may comprise at least one active heating element that is disposed behind the reflection surface of the mirror at the mirror base. In this way, the central region of a reflecting surface can also be heated if this is desirable for compensation purposes.
Alternatively or additionally, a cooling device for the mirror and/or the lens may also be provided to additionally compensate for the illumination-induced imaging changes. In principle, the same applies to the effect of the cooling device as already explained above in relation to the heating device. A defined dimensional change in the optical surfaces that can be used to compensate additionally for illumination-induced imaging changes can also be achieved by cooling.
The cooling device may have at least one thermal contact surface for dissipating the illumination-induced heating of the mirror and/or the lens. Such a thermal contact surface may be disposed either in the region of the circumferential surface or, in the case of a mirror, also behind the reflection surface of the mirror on the mirror base. The heat dissipation achieves a passive cooling of those regions of the optical components adjacent to the thermal contact surface. This can also be used to compensate for the illumination-induced imaging changes.
Analogously to the heating device, the cooling device may also comprise at least one cooling element that is disposed in a first embodiment at the circumference of the lens and/or of the mirror and in a second embodiment, alternatively or additionally, behind the reflection surface of the mirror at the mirror base. Here the same applies as was explained above for the analogously designed heating device.